Conventional precision voltage references (PVRs) are typically Zener diodes run in reverse breakdown, where relatively large changes in operating current produce relatively small changes in operating voltage. A significant disadvantage of these traditional PVRs is that they must be very carefully doped to be useful as radiation hard PVRs, which are required components of missile guidance systems.
Negative resistance devices (NRDs), for example tunnel diodes and quantum well devices, have local extrema in their I-V curves which make them excellent candidates for use as PVRs. These NRDs are often very radiation hard without the use of special radiation hardening fabrication techniques. This makes them ideally suited for use as PVRs for missile guidance systems. However, NRDs are inherently unstable around the inflection points (local extrema) in their curve due to the negative resistance characteristics around these operating points. Thus, although well suited for use as radiation hard PVRs, NRDs have inherent problems which have kept them from being developed for use as PVRs in radiation hard missle guidance systems.
Typically, the local extremum of an NRD, whether it be a local current extremum or a local voltage extremum, is found by manually plotting the I-V curve for the device. The NRD is fed a signal with a known voltage or current, and the resulting current or voltage, respectively, is measured. The voltage or current is then increased in very small incremental steps until an inflection point is found. Since these devices have at least two inflection points, one at either end of the negative resistance portion of the curve, this manual measure and plot technique is extremely time consuming, and must be repeated whenever there is drift in the device which causes a shift in the position of a local extremum.
This manual technique cannot be used with PVRs in missile guidance systems because the PVR characteristics change with time, temperature, and radiation. As an example, an NRD device with an extremum in current has a stable voltage at this current extremum. The current at this extreme, however, is not stable. The only way to determine the reference voltage is by determining the location of the current inflection point and not by determining the absolute value of current.
A computerized version of the manual curve plotting technique described above can be used to determine the inflection point of an NRD PVR in a missile guidance system. The weakness of that approach is that the computer must plot many points from a broad region around the NRD PVR inflection point and deal with a very large amount of data. This requires a relatively large amount of circuitry, and is a relatively slow technique.
NRDs are also frequently used as triggering devices. For example, tunnel diodes are used to trigger oscilloscope sweeps on detection of a signal of a predetermined size. Typically, the NRD is biased somewhat below the positive going peak that defines the beginning of the negative resistance region of the NRD. When a signal larger than the difference between the bias level and the peak is sensed, the NRD switches to its second stable state, that is, beyond the negative going peak that defines the end of the negative resistance region. This switching action then triggers an event, such as an oscilloscope sweep or an indication of the detection of a signal of at least a predetermined size.
Although these triggering devices employ NRDs, the NRDs are not used as PVRs. They are merely used as switches. In fact, even if these NRDs could be held at their peaks, they would not be useful as triggers because even an extremely small noise signal would trigger the NRD and thus give a false indication of a significant event.
As a result of the inherent difficulty of "stabilizing" an NRD at its peak so it can be used as a PVR, NRDs have not been developed for use as voltage references. However, because NRDs are extremely radiation hard and peak at useful voltages, typically more than 0.06 volts, they are ideally suited for use as PVRs in missile guidance systems.